There has been much recent interest and development in electronic devices incorporating a functional metal oxide layer. Perovskite thin films are particularly useful because many perovskite materials exhibit non-linear behavior such as ferroelectricity or have electrical characteristics that depend on other parameters such that they can be used for sensors or actuators.
One such application is a non-volatile ferroelectric memory as I have described in U.S. Pat. No. 6,518,609, incorporated herein by reference in its entirety. The fundamental structure of a ferroelectric memory includes a thin film of ferroelectric material sandwiched between the electrodes of a capacitor. The ferroelectric material has two polarization states. Once the ferroelectric thin film has been poled into one of the two states, it remains in that state without further application of power. Furthermore, the ferroelectric state can be electrically tested. As a result, a ferroelectric capacitor can act as a non-volatile memory. One of the challenges of commercially important ferroelectric memories addressed in the above cited patent is integrating a large number of such ferroelectric cells on a silicon substrate.
Another important application of perovskite thin films includes micro electromechanical system (MEMS) devices. MEMS technology borrows the fabrication techniques developed for silicon integrated circuits to form small mechanical devices in a silicon layer of a substrate that can move and interact with electrical signals. There are several forms of electromechanical actuation. In one form, a piezoelectric layer is formed over a thin cantilevered silicon layer and is connected between two electrical terminals. A voltage applied across the piezoelectric layer causes it and the silicon layer to flex in a predetermined way. Such a structure has been used to form arrays of movable micromirrors for a communication switch. In a complementary form, the structure can act as a pressure sensor. A pressure differential across the cantilevered structure will cause it to flex. The distortion can be electrically detected as a voltage across the piezoelectric layer in a mechanism similar to a microphone. Again, the challenge is to integrate onto a silicon substrate a significant number of such devices including the piezoelectric layer.
Ferroelectric memories and piezoelectric MEMS structures often use a functional metal oxide layer of a perovskite material. One of the most popular perovskite material for these applications is lead zirconium titanate (PbZrxTi1-xO3 or PZT) and its related alloys of lead lanthanum niobium zirconium titanate (PLNZT). It is understood the composition of commercially important perovskite materials is often not strictly stoichiometric. There are other devices which incorporate these and similar perovskite materials, such as non-volatile field effect transistors, pyroelectric infrared sensors, other optical devices, pyrometers, and static ferroelectric RAMs.
Almost all these devices benefit from a high quality perovskite film having well defined crystallinity. In U.S. Pat. No. 6,432,546, I and others have disclosed growing high quality PZT on a layer strontium titanate (SrTiO3 or STO) grown over silicon. In U.S. Pat. Nos. 6,518,609 and 6,781,176, I have described doping the STO to be conductive. I and Schlom in U.S. Pat. No. 6,642,539, incorporated herein by reference in its entirety, have disclosed that the doping elements in STO can be chosen from different substituents.
The use of PZT as the functional metal oxide layer presents some difficulties. It contains a high fraction of lead. Lead is toxic and thus raises environmental issues both during fabrication and for disposal of old devices. Furthermore, lead is considered a heavy metal contaminant in the fabrication of silicon integrated circuits and it may introduce reliability problems in semiconductor circuits.
It would be preferred if functional metal oxide layers exhibiting ferroelectricity, piezoelectricity, and other qualities associated with perovskites have a composition not including lead.